Hereinafter, a “Q” prefix in a word or phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used.
Quantum mechanics is a branch of physics that describes how the physical world works at the most fundamental level. According to quantum mechanics, objects can behave in ways that are counterintuitive, such as being in more than one state at the same time (superposition) and being entangled with other objects. Quantum computing harnesses these quantum phenomena to process information.
The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a conventional processor fabricated using semiconductor materials and technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0.
A quantum processor (Q-processor) uses the unintuitive nature of entangled qubit devices (compactly referred to herein as “qubit,” plural “qubits”) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states—such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where binary computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing.
Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These 1s and 0s act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two key principles of quantum physics: superposition and entanglement. Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually.
Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor using superconducting qubits (IBM is a registered trademark of International Business Machines Corporation in the United States and in other countries.)
A superconducting qubit includes a Josephson junction. A Josephson junction is formed by separating two superconducting metal layers by a thin insulating material. When the metal in the superconducting layers is caused to become superconducting—e.g. by reducing the temperature of the metal to a specified cryogenic temperature—pairs of electrons can tunnel from one superconducting layer through the insulating layer to the other superconducting layer. The equations describing this process are identical to that of a non-dissipative nonlinear inductor. In a qubit, the Josephson junction is electrically coupled in parallel with one or more capacitive devices forming a nonlinear microwave oscillator. The oscillator has a resonance/transition frequency determined by the value of the Josephson junction inductance and the parallel capacitance. Any reference to the term “qubit” is a reference to a superconducting qubit oscillator circuitry that employs a Josephson junction unless expressly distinguished where used.
The information processed by qubits is carried or transmitted in the form of microwave signals/photons in the range of microwave frequencies. The microwave frequency of a qubit output is determined by the resonance frequency of the qubit. The microwave signals are captured, processed, and analyzed to decipher the quantum information encoded therein. A readout circuit is a circuit coupled with the qubit to capture, read, and measure the quantum state of the qubit. An output of the readout circuit is information usable by a Q-processor to perform computations.
A superconducting qubit has two quantum states—|0> and |1>. These two states may be two energy states of atoms, for example, the ground (|0>) and first excited state (|1>) of a superconducting artificial atom (superconducting qubit). Other examples include spin-up and spin-down of the nuclear or electronic spins, two positions of a crystalline defect, and two states of a quantum dot. Since the system is of a quantum nature, any combination of the two states is allowed and valid.
For quantum computing using qubits to be reliable, quantum circuits, e.g., the qubits themselves, the readout circuitry associated with the qubits, and other parts of the quantum processor, must not alter the energy states of the qubit, such as by injecting or dissipating energy, in any significant manner or influence the relative phase between the |0> and |1> states of the qubit. This operational constraint on any circuit that operates with quantum information necessitates special considerations in fabricating semiconductor and superconducting structures that are used in such circuits.
The illustrative embodiments recognize that a qubit's resonance frequency is inherently fixed at the time the qubit is fabricated, i.e., when the Josephson Junction and the capacitive element of the qubit-oscillator are fabricated on a Q-processor chip. The illustrative embodiments further recognize that in the simplest implementation of a quantum processor (Q-processor), at least two qubits are needed to implement a quantum logic gate (Q-gate). Therefore, a Q-processor chip is typically fabricated to have at least 2, but often 8, 16, hundreds, thousands, or more qubits on a single Q-processor chip. The illustrative embodiments recognize that a fixed-frequency microwave resonator (bus) connects neighboring qubits such that the appropriate microwave signals can be passed between qubits to activate a Q-gate.
Some qubits are fixed-frequency qubits, i.e., their resonance frequencies are not changeable. Other qubits are frequency-tunable qubits. A Q-processor can employ fixed-frequency qubits, frequency-tunable qubits, or a combination thereof.
The illustrative embodiments recognize that when the resonance frequencies of two neighboring coupled qubits on a chip are the same or within a threshold band of frequencies or their higher transition frequencies are on resonance or close to resonance, then negative effects can happen such as, crosstalk, quantum decoherence, energy decay, creation of mixed states, unintended information transfer, quantum state leakage and so on. Having such qubits can also negatively affect the performance or utility of certain quantum gates such as cross-resonance gates which have stringent requirements on the spectrum of resonance frequencies of qubits upon which the gate is operating on. Therefore, the illustrative embodiments recognize that one challenge in quantum processors that are based on coupled fixed-frequency qubits is frequency crowding or frequency collision between adjacent qubits, in particular, when cross-resonance gates are used.
It is important to note that while the proposed tuning technique is motivated by the need to solve frequency collisions of coupled qubits on the same chip which are acted on with cross-resonance gates, the proposed tuning technique is general, and can be applied to other kinds of quantum devices on chip which require gates based on microwave-driven qubits.
The illustrative embodiments recognize that a fixed-frequency qubit (hereinafter compactly referred to as a “fixed qubit”) is designed to be fixed in frequency to be immune to flux noise. The illustrative embodiments recognize that one challenge in quantum processors that are based on fixed-frequency qubits is low on/off ratios between when microwave signals turn on an interaction (on interaction strength) and the interactions between neighboring qubits when these signals are disabled (off interaction strength). The illustrative embodiments further recognize that one challenge in quantum processors that are based on fixed-frequency qubits is enabling a gate of interest without producing unwanted interactions at other gates.